These bottles will hold at least 120 PSI (8 bar) of pressure. I've only
launched it at 6 bar due to lack of an adequate pump (and I don't have a
remote launcher yet - see below), but even at that I have trouble finding
a park big enough to be safe - an Australian football field is huge by the
standards of other football codes, but isn't big enough if the rocket is
even slightly unstable at launch - I've had to walk a long way to retrieve
Mal Goris's pages, where he describes his development of my
launcher with detailed photos and diagrams.
- The nozzle
I use a plastic garden-hose connector for the nozzle.
Cutting it down to the right size and fitting it into the
original cap of the bottle is tricky, but works well.
I use a 19mm spade drill with two teeth on the outer edge to drill
a hole in the centre of the bottle top. Most of the hose attachment
must be cut away to leave a thin lip only 2mm wide to fit inside the
bottle top. Inside that you can use the soft plastic sealer (with
the centre cut out) as a washer. I usually glue the nozzle into the
bottle top, but that just stops them getting separated, it isn't
enough to stop it leaking.
- The fin
Because the water is expelled more slowly than with an open-mouth
launcher, during the early phase of the flight the bottle would be
very unstable, since the weight is in the back (rockets need their
weight concentrated forward of the centre of pressure). All
my experiments with three fins failed until I came up with the
cylindrical fin shown in the pictures here.
Note how far the
fin trails behind the rocket - this is essential.
- The launcher
Use of the hose connector as a nozzle immediately suggests the use of
the matching part to pressurise and release the rocket. The launcher
shown has a comfortable pistol grip, string-pull trigger, and consists
of a piece of plastic hose joined using a hose clamp to a copper pipe
with a 90 degree angle piece on the end, joined to a 1/4 inch tube for
attachment of an air pump.
The trigger string is a piece of blind cord, tied around the hose-end
tightly and overlapped so as to get the two loose ends to come out on
opposite sides, as shown below. You can remember some of that
boy-scout stuff, can't you? I passed the string through a plastic cap
attached to the copper pipe to stop the ends disappearing to where I
can't reach them, and attached a loop for a trigger.
- The nose cone
The bottom of the bottle has the top of another taped to it to act
as a nose cone, and to prevent the hard landings from weakening the
high-pressure part. I have had a bottle explode after twenty
landings - huge bang - louder than a
shotgun! I heard later the local police were called to
investigate... Don't hold the bottle while pumping! There
was a report during the 1997 test cricket series in South Africa of
someone having three fingers sewn back on after a dry-ice bottle
grenade blew up while they were holding it. Apparently the grenade
(a capped bottle with some dry ice inside) was being thrown around
when someone got unlucky. However, the plastic doesn't fragment -
just tears open and turns inside-out.
- New nose cones!
See also the New nose cones of my Guppy rockets.
Closeups of launcher
That's the metal part of an old car tyre valve, threaded and soldered
into a brass end-cap. I need to add a pressure gauge to this soon.
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